git-apply(1)

Reads a "diff -up1" or git generated patch file and applies it to the working tree.

git-checkout-index(1)

Copy files from the index to the working tree.

git-commit-tree(1)

Creates a new commit object.

git-hash-object(1)

Computes the object ID from a file.

git-index-pack(1)

Build pack idx file for an existing packed archive.

git-init-db(1)

Creates an empty git object database, or reinitialize an existing one.

git-merge-index(1)

Runs a merge for files needing merging.

git-mktag(1)

Creates a tag object.

git-pack-objects(1)

Creates a packed archive of objects.

git-prune-packed(1)

Remove extra objects that are already in pack files.

git-read-tree(1)

Reads tree information into the index.

git-repo-config(1)

Get and set options in .git/config.

git-unpack-objects(1)

Unpacks objects out of a packed archive.

git-update-index(1)

Registers files in the working tree to the index.

git-write-tree(1)

Creates a tree from the index.

git-cat-file(1)

Provide content or type/size information for repository objects.

git-describe(1)

Show the most recent tag that is reachable from a commit.

git-diff-index(1)

Compares content and mode of blobs between the index and repository.

git-diff-files(1)

Compares files in the working tree and the index.

git-diff-stages(1)

Compares two "merge stages" in the index.

git-diff-tree(1)

Compares the content and mode of blobs found via two tree objects.

git-fsck-objects(1)

Verifies the connectivity and validity of the objects in the database.

git-ls-files(1)

Information about files in the index and the working tree.

git-ls-tree(1)

Displays a tree object in human readable form.

git-merge-base(1)

Finds as good common ancestors as possible for a merge.

git-name-rev(1)

Find symbolic names for given revs.

git-pack-redundant(1)

Find redundant pack files.

git-rev-list(1)

Lists commit objects in reverse chronological order.

git-show-index(1)

Displays contents of a pack idx file.

git-tar-tree(1)

Creates a tar archive of the files in the named tree object.

git-unpack-file(1)

Creates a temporary file with a blob's contents.

git-var(1)

Displays a git logical variable.

git-verify-pack(1)

Validates packed git archive files.

In general, the interrogate commands do not touch the files in the working tree.

git-clone-pack(1)

Clones a repository into the current repository (engine for ssh and local transport).

git-fetch-pack(1)

Updates from a remote repository (engine for ssh and local transport).

git-http-fetch(1)

Downloads a remote git repository via HTTP by walking commit chain.

git-local-fetch(1)

Duplicates another git repository on a local system by walking commit chain.

git-peek-remote(1)

Lists references on a remote repository using upload-pack protocol (engine for ssh and local transport).

git-receive-pack(1)

Invoked by git-send-pack to receive what is pushed to it.

git-send-pack(1)

Pushes to a remote repository, intelligently.

git-http-push(1)

Push missing objects using HTTP/DAV.

git-shell(1)

Restricted shell for GIT-only SSH access.

git-ssh-fetch(1)

Pulls from a remote repository over ssh connection by walking commit chain.

git-ssh-upload(1)

Helper "server-side" program used by git-ssh-fetch.

git-update-server-info(1)

Updates auxiliary information on a dumb server to help clients discover references and packs on it.

git-upload-pack(1)

Invoked by git-clone-pack and git-fetch-pack to push what are asked for.

These environment variables apply to all core git commands. Nb: it is worth noting that they may be used/overridden by SCMS sitting above git so take care if using Cogito etc.

GIT_INDEX_FILE

This environment allows the specification of an alternate index file. If not specified, the default of $GIT_DIR/index is used.

GIT_OBJECT_DIRECTORY

If the object storage directory is specified via this environment variable then the sha1 directories are created underneath - otherwise the default $GIT_DIR/objects directory is used.

GIT_ALTERNATE_OBJECT_DIRECTORIES

Due to the immutable nature of git objects, old objects can be archived into shared, read-only directories. This variable specifies a ":" separated list of git object directories which can be used to search for git objects. New objects will not be written to these directories.

GIT_DIR

If the GIT_DIR environment variable is set then it specifies a path to use instead of the default .git for the base of the repository.

GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL

see git-commit-tree(1)

GIT_DIFF_OPTS, GIT_EXTERNAL_DIFF

see the "generating patches" section in : git-diff-index(1); git-diff-files(1); git-diff-tree(1)

The object database is literally just a content-addressable collection of objects. All objects are named by their content, which is approximated by the SHA1 hash of the object itself. Objects may refer to other objects (by referencing their SHA1 hash), and so you can build up a hierarchy of objects.

All objects have a statically determined "type" aka "tag", which is determined at object creation time, and which identifies the format of the object (i.e. how it is used, and how it can refer to other objects). There are currently four different object types: "blob", "tree", "commit" and "tag".

A "blob" object cannot refer to any other object, and is, like the type implies, a pure storage object containing some user data. It is used to actually store the file data, i.e. a blob object is associated with some particular version of some file.

A "tree" object is an object that ties one or more "blob" objects into a directory structure. In addition, a tree object can refer to other tree objects, thus creating a directory hierarchy.

A "commit" object ties such directory hierarchies together into a DAG of revisions - each "commit" is associated with exactly one tree (the directory hierarchy at the time of the commit). In addition, a "commit" refers to one or more "parent" commit objects that describe the history of how we arrived at that directory hierarchy.

As a special case, a commit object with no parents is called the "root" object, and is the point of an initial project commit. Each project must have at least one root, and while you can tie several different root objects together into one project by creating a commit object which has two or more separate roots as its ultimate parents, that's probably just going to confuse people. So aim for the notion of "one root object per project", even if git itself does not enforce that.

A "tag" object symbolically identifies and can be used to sign other objects. It contains the identifier and type of another object, a symbolic name (of course!) and, optionally, a signature.

Regardless of object type, all objects share the following characteristics: they are all deflated with zlib, and have a header that not only specifies their type, but also provides size information about the data in the object. It's worth noting that the SHA1 hash that is used to name the object is the hash of the original data plus this header, so sha1sum file does not match the object name for file. (Historical note: in the dawn of the age of git the hash was the sha1 of the compressed object.)

As a result, the general consistency of an object can always be tested independently of the contents or the type of the object: all objects can be validated by verifying that (a) their hashes match the content of the file and (b) the object successfully inflates to a stream of bytes that forms a sequence of <ascii type without space> + <space> + <ascii decimal size> + <byte\0> + <binary object data>.

The structured objects can further have their structure and connectivity to other objects verified. This is generally done with the git-fsck-objects program, which generates a full dependency graph of all objects, and verifies their internal consistency (in addition to just verifying their superficial consistency through the hash).

The object types in some more detail:

A "blob" object is nothing but a binary blob of data, and doesn't refer to anything else. There is no signature or any other verification of the data, so while the object is consistent (it is indexed by its sha1 hash, so the data itself is certainly correct), it has absolutely no other attributes. No name associations, no permissions. It is purely a blob of data (i.e. normally "file contents").

In particular, since the blob is entirely defined by its data, if two files in a directory tree (or in multiple different versions of the repository) have the same contents, they will share the same blob object. The object is totally independent of its location in the directory tree, and renaming a file does not change the object that file is associated with in any way.

A blob is typically created when git-update-index(1) is run, and its data can be accessed by git-cat-file(1).

The next hierarchical object type is the "tree" object. A tree object is a list of mode/name/blob data, sorted by name. Alternatively, the mode data may specify a directory mode, in which case instead of naming a blob, that name is associated with another TREE object.

Like the "blob" object, a tree object is uniquely determined by the set contents, and so two separate but identical trees will always share the exact same object. This is true at all levels, i.e. it's true for a "leaf" tree (which does not refer to any other trees, only blobs) as well as for a whole subdirectory.

For that reason a "tree" object is just a pure data abstraction: it has no history, no signatures, no verification of validity, except that since the contents are again protected by the hash itself, we can trust that the tree is immutable and its contents never change.

So you can trust the contents of a tree to be valid, the same way you can trust the contents of a blob, but you don't know where those contents came from.

Side note on trees: since a "tree" object is a sorted list of "filename+content", you can create a diff between two trees without actually having to unpack two trees. Just ignore all common parts, and your diff will look right. In other words, you can effectively (and efficiently) tell the difference between any two random trees by O(n) where "n" is the size of the difference, rather than the size of the tree.

Side note 2 on trees: since the name of a "blob" depends entirely and exclusively on its contents (i.e. there are no names or permissions involved), you can see trivial renames or permission changes by noticing that the blob stayed the same. However, renames with data changes need a smarter "diff" implementation.

A tree is created with git-write-tree(1) and its data can be accessed by git-ls-tree(1). Two trees can be compared with git-diff-tree(1).

The "commit" object is an object that introduces the notion of history into the picture. In contrast to the other objects, it doesn't just describe the physical state of a tree, it describes how we got there, and why.

A "commit" is defined by the tree-object that it results in, the parent commits (zero, one or more) that led up to that point, and a comment on what happened. Again, a commit is not trusted per se: the contents are well-defined and "safe" due to the cryptographically strong signatures at all levels, but there is no reason to believe that the tree is "good" or that the merge information makes sense. The parents do not have to actually have any relationship with the result, for example.

Note on commits: unlike real SCM's, commits do not contain rename information or file mode change information. All of that is implicit in the trees involved (the result tree, and the result trees of the parents), and describing that makes no sense in this idiotic file manager.

A commit is created with git-commit-tree(1) and its data can be accessed by git-cat-file(1).

An aside on the notion of "trust". Trust is really outside the scope of "git", but it's worth noting a few things. First off, since everything is hashed with SHA1, you can trust that an object is intact and has not been messed with by external sources. So the name of an object uniquely identifies a known state - just not a state that you may want to trust.

Furthermore, since the SHA1 signature of a commit refers to the SHA1 signatures of the tree it is associated with and the signatures of the parent, a single named commit specifies uniquely a whole set of history, with full contents. You can't later fake any step of the way once you have the name of a commit.

So to introduce some real trust in the system, the only thing you need to do is to digitally sign just one special note, which includes the name of a top-level commit. Your digital signature shows others that you trust that commit, and the immutability of the history of commits tells others that they can trust the whole history.

In other words, you can easily validate a whole archive by just sending out a single email that tells the people the name (SHA1 hash) of the top commit, and digitally sign that email using something like GPG/PGP.

To assist in this, git also provides the tag object...

Git provides the "tag" object to simplify creating, managing and exchanging symbolic and signed tokens. The "tag" object at its simplest simply symbolically identifies another object by containing the sha1, type and symbolic name.

However it can optionally contain additional signature information (which git doesn't care about as long as there's less than 8k of it). This can then be verified externally to git.

Note that despite the tag features, "git" itself only handles content integrity; the trust framework (and signature provision and verification) has to come from outside.

A tag is created with git-mktag(1), its data can be accessed by git-cat-file(1), and the signature can be verified by git-verify-tag(1).

You update the index with information from the working directory with the git-update-index(1) command. You generally update the index information by just specifying the filename you want to update, like so:

git-update-index filename

but to avoid common mistakes with filename globbing etc, the command will not normally add totally new entries or remove old entries, i.e. it will normally just update existing cache entries.

To tell git that yes, you really do realize that certain files no longer exist, or that new files should be added, you should use the --remove and --add flags respectively.

NOTE! A --remove flag does not mean that subsequent filenames will necessarily be removed: if the files still exist in your directory structure, the index will be updated with their new status, not removed. The only thing --remove means is that update-cache will be considering a removed file to be a valid thing, and if the file really does not exist any more, it will update the index accordingly.

As a special case, you can also do git-update-index --refresh, which will refresh the "stat" information of each index to match the current stat information. It will not update the object status itself, and it will only update the fields that are used to quickly test whether an object still matches its old backing store object.

You write your current index file to a "tree" object with the program

git-write-tree

that doesn't come with any options - it will just write out the current index into the set of tree objects that describe that state, and it will return the name of the resulting top-level tree. You can use that tree to re-generate the index at any time by going in the other direction:

You read a "tree" file from the object database, and use that to populate (and overwrite - don't do this if your index contains any unsaved state that you might want to restore later!) your current index. Normal operation is just

git-read-tree <sha1 of tree>

and your index file will now be equivalent to the tree that you saved earlier. However, that is only your index file: your working directory contents have not been modified.

You update your working directory from the index by "checking out" files. This is not a very common operation, since normally you'd just keep your files updated, and rather than write to your working directory, you'd tell the index files about the changes in your working directory (i.e. git-update-index).

However, if you decide to jump to a new version, or check out somebody else's version, or just restore a previous tree, you'd populate your index file with read-tree, and then you need to check out the result with

git-checkout-index filename

or, if you want to check out all of the index, use -a.

NOTE! git-checkout-index normally refuses to overwrite old files, so if you have an old version of the tree already checked out, you will need to use the "-f" flag (before the "-a" flag or the filename) to force the checkout.

Finally, there are a few odds and ends which are not purely moving from one representation to the other:

To commit a tree you have instantiated with "git-write-tree", you'd create a "commit" object that refers to that tree and the history behind it - most notably the "parent" commits that preceded it in history.

Normally a "commit" has one parent: the previous state of the tree before a certain change was made. However, sometimes it can have two or more parent commits, in which case we call it a "merge", due to the fact that such a commit brings together ("merges") two or more previous states represented by other commits.

In other words, while a "tree" represents a particular directory state of a working directory, a "commit" represents that state in "time", and explains how we got there.

You create a commit object by giving it the tree that describes the state at the time of the commit, and a list of parents:

git-commit-tree <tree> -p <parent> [-p <parent2> ..]

and then giving the reason for the commit on stdin (either through redirection from a pipe or file, or by just typing it at the tty).

git-commit-tree will return the name of the object that represents that commit, and you should save it away for later use. Normally, you'd commit a new HEAD state, and while git doesn't care where you save the note about that state, in practice we tend to just write the result to the file pointed at by .git/HEAD, so that we can always see what the last committed state was.

Here is an ASCII art by Jon Loeliger that illustrates how various pieces fit together.

commit-tree commit obj +----+ | | | | V V +-----------+ | Object DB | | Backing | | Store | +-----------+ ^ write-tree | | tree obj | | | | read-tree | | tree obj V +-----------+ | Index | | "cache" | +-----------+ update-index ^ blob obj | | | | checkout-index -u | | checkout-index stat | | blob obj V +-----------+ | Working | | Directory | +-----------+

You can examine the data represented in the object database and the index with various helper tools. For every object, you can use git-cat-file(1) to examine details about the object:

git-cat-file -t <objectname>

shows the type of the object, and once you have the type (which is usually implicit in where you find the object), you can use

git-cat-file blob|tree|commit|tag <objectname>

to show its contents. NOTE! Trees have binary content, and as a result there is a special helper for showing that content, called git-ls-tree, which turns the binary content into a more easily readable form.

It's especially instructive to look at "commit" objects, since those tend to be small and fairly self-explanatory. In particular, if you follow the convention of having the top commit name in .git/HEAD, you can do

git-cat-file commit HEAD

to see what the top commit was.

Git helps you do a three-way merge, which you can expand to n-way by repeating the merge procedure arbitrary times until you finally "commit" the state. The normal situation is that you'd only do one three-way merge (two parents), and commit it, but if you like to, you can do multiple parents in one go.

To do a three-way merge, you need the two sets of "commit" objects that you want to merge, use those to find the closest common parent (a third "commit" object), and then use those commit objects to find the state of the directory ("tree" object) at these points.

To get the "base" for the merge, you first look up the common parent of two commits with

git-merge-base <commit1> <commit2>

which will return you the commit they are both based on. You should now look up the "tree" objects of those commits, which you can easily do with (for example)

git-cat-file commit <commitname> | head -1

since the tree object information is always the first line in a commit object.

Once you know the three trees you are going to merge (the one "original" tree, aka the common case, and the two "result" trees, aka the branches you want to merge), you do a "merge" read into the index. This will complain if it has to throw away your old index contents, so you should make sure that you've committed those - in fact you would normally always do a merge against your last commit (which should thus match what you have in your current index anyway).

To do the merge, do

git-read-tree -m -u <origtree> <yourtree> <targettree>

which will do all trivial merge operations for you directly in the index file, and you can just write the result out with git-write-tree.

Historical note. We did not have -u facility when this section was first written, so we used to warn that the merge is done in the index file, not in your working tree, and your working tree will not match your index after this step. This is no longer true. The above command, thanks to -u option, updates your working tree with the merge results for paths that have been trivially merged.

Sadly, many merges aren't trivial. If there are files that have been added.moved or removed, or if both branches have modified the same file, you will be left with an index tree that contains "merge entries" in it. Such an index tree can NOT be written out to a tree object, and you will have to resolve any such merge clashes using other tools before you can write out the result.

You can examine such index state with git-ls-files --unmerged command. An example:

$ git-read-tree -m $orig HEAD $target $ git-ls-files --unmerged 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c

Each line of the git-ls-files --unmerged output begins with the blob mode bits, blob SHA1, stage number, and the filename. The stage number is git's way to say which tree it came from: stage 1 corresponds to $orig tree, stage 2 HEAD tree, and stage3 $target tree.

Earlier we said that trivial merges are done inside git-read-tree -m. For example, if the file did not change from $orig to HEAD nor $target, or if the file changed from $orig to HEAD and $orig to $target the same way, obviously the final outcome is what is in HEAD. What the above example shows is that file hello.c was changed from $orig to HEAD and $orig to $target in a different way. You could resolve this by running your favorite 3-way merge program, e.g. diff3 or merge, on the blob objects from these three stages yourself, like this:

$ git-cat-file blob 263414f... >hello.c~1 $ git-cat-file blob 06fa6a2... >hello.c~2 $ git-cat-file blob cc44c73... >hello.c~3 $ merge hello.c~2 hello.c~1 hello.c~3

This would leave the merge result in hello.c~2 file, along with conflict markers if there are conflicts. After verifying the merge result makes sense, you can tell git what the final merge result for this file is by:

mv -f hello.c~2 hello.c
git-update-index hello.c

When a path is in unmerged state, running git-update-index for that path tells git to mark the path resolved.

The above is the description of a git merge at the lowest level, to help you understand what conceptually happens under the hood. In practice, nobody, not even git itself, uses three git-cat-file for this. There is git-merge-index program that extracts the stages to temporary files and calls a "merge" script on it:

git-merge-index git-merge-one-file hello.c

and that is what higher level git resolve is implemented with.